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Cats are the leading human-related cause of wildlife death in the United States. This is the conclusion of a study published earlier this year by scientists at the Smithsonian Institution and U.S. Fish and Wildlife Service.

The authors of the study reviewed 21 publications that estimated free-ranging cat predation in the western world and then developed a mathematical model (which they believe is conservative) to estimate total effects on wildlife. Pete Marra, the senior author on the study, is quoted by the Journal of the American Veterinary Medical Association as being “absolutely stunned by the results.”

Cats in the United States are estimated to kill 1.4 billion to 3.7 billion birds each year. In the same time frame they are estimated to kill 6.9 billion to 20.7 billion mammals, 258 million to 822 million reptiles, and 95 million to 299 million amphibians. The conclusion is that cats are having population-level effects on American wildlife.

The authors conclude these numbers mean cats kill more wildlife than building strikes, vehicular trauma, habitat loss, and agriculture. And they point the finger of culpability squarely at one group: feral cats.

The authors estimate that feral cats are responsible for 69 percent of the bird mortality and 89 percent of the mammal mortality caused by cats overall.

The authors don’t offer huge amounts of conjecture on what to do about the problem (although they do indicate that, as always, more research is needed). They state, “Scientifically sound conservation and policy intervention is needed to reduce this impact.” But they don’t state what the policy intervention should be.

Predictably, the study has led to controversy. Many cat lovers worry that the study will cause some folks to paint all cats with the same brush, leading to decreased cat adoptions from shelters. Feral cat advocates worry that the study will be used to change feral cat population management strategies away from trap-neuter-release, or TNR, and toward trap and kill.

And frankly, with one exception, I’m not sure what to do with this information either. Here is that one exception: I recommend that you keep your cat indoors.

I have always recommended keeping cats indoors for their own safety. Outdoor cats get lost, get hit by cars, get in fights, and contract diseases. They also have the potential to kill wildlife. And that potential gives ammunition to people who want to kill feral cats. Don’t give those people that ammunition.

I also am not sure I trust the mathematical model used by the researchers. I’m not familiar with the details of the model, but my armchair studies of economics have made me generally skeptical about mathematical models (such as those promulgated by Moody’s that claimed mortgage-backed securities were as safe as U.S. government bonds). The numbers of wild animals killed by cats are simply estimates. More research is needed.

Also, remember that the study is claiming that feral cats — not house cats — are the real problem. The big concern is that people will start to call for the wholesale elimination of feral cats. If you’re for the wholesale elimination of feral cats, I hate to break this to you: It’s not going to happen.

There are examples of small islands where feral cats drove some species of birds to near-extinction. When the feral cat populations on those islands were eliminated (which is to say, killed off), the birds rebounded. Removal of all feral cats from an island is ethically dubious, but at least is is possible.

The United States is not a small island. I know you’re never supposed to say never, but I’m going to do it anyway. Feral cats will never be eliminated from the United States. Cats are reproducers, and they are survivors. There will be feral cats long after humanity has wiped itself out. Trying to eliminate all feral cats is a futile exercise.

Last week I spoke of the remarkable fecundity of cats. In that context, I stated that ecosystem carrying capacities alone determine cat populations. Wholesale slaughter of ferals would reduce their numbers for as long as the slaughter continued. There would be fewer cats during that time, but the ecosystem would be ripe for new arrivals. The feral cats would be back to their previous numbers as soon as the efforts stopped.

Also, we should look at which animals are being killed by cats. How many of those mammals are urban or suburban mice and rats? Does anybody really miss them? And let’s face another unpleasant fact: Any species of bird or mammal that could not withstand cat predation went extinct years ago.

So, what should be done about the problem of feline wildlife destruction? I’m not sure that anything can be done. But I do know one thing: I don’t support the wholesale slaughter of feral cats.

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